1. Field of the Invention
The present invention concerns a device for stirring or pumping, wherein a magnetic rotor of suitable configuration is free to rotate in a receptacle or tube and is driven by a rotary electromagnetic field arrangement.
The particles of liquid or gas are put into circular motion by stirring, whereby concentration and temperature differentials can be speedily eliminated. With the aid of the stirring process solids and gases are more quickly dissolved and in extractive processes stirring helps to accelerate the mass-transfer and also has a very important part to play in completing chemical reactions: insufficient stirring can deteriorate product composition and yield and may even result in explosions.
2. Description of the Prior Art
Pumping induces mainly linear directed motion in gases or liquids, which phenomemon is typically made use of in moving the medium from one receptacle to another. In some applications, such as in loop reactors of the type used in chemical processes, for example, pumps are also used to produce the same effects as stirring, whereby the medium is circulated from the output side of the pump and returned to the suction side.
This is the reason why a universal device has been sought which is capable of inducing universal movement in the medium being handled, allowing a number of different functions, such as stirring and pumping to be carried out by the use of rotors of suitable configuration, in conjunction with further functions such as gas absorption, for example. Furthermore, it is important that such movement in the medium be induced in receptacles or vessels of various configurations and/or at various different stages in the course of the process or locations in the apparatus employed in the process; in other words, to achieve universal applicability.
Stirrers in which a permanently magnetic stirrer or paddle device is driven by a rotary magnetic field are in widespread use, principally in chemical laboratories in processes where only small quantities of liquid have to be stirred. With previously known stirrers of this type, the rotary magnetic field has been generated by the rotation of a permanent magnet centrally mounted on the spindle of an electric motor.
In drive systems of the conventional type, the presence of mechanical moving components has an adverse effect on operating reliability, especially in continuous service where operation cannot be supervised at all times. The design of these systems requires that they generally occupy a great deal of room and they are also extremely heavy, which is a further disadvantage if they are to be used in conjunction with fragile and complicated glass apparatus and in applications where space is restricted, as is often the case with optical measurement instruments, for example. The provision of arrangements to govern the speed of the driving motor is complex and expensive, as is the design of leakproof vessels and corrosion resisting stirring devices for use in thermostatically controlled baths. Furthermore, conventional stirring mechanisms can only be arranged in an offset location in a stirring vessel with a centrally arranged outlet. As a result the amount of power which can be applied will be considerably diminished.
Various proposals are known (Swiss patent document No. CH 501 429, for example) for eliminating some of these disadvantages, by arranging for the rotary magnetic field to be generated by fixed electromagnets, for example.
In some of the designs which have been proposed, there is a tendency for the stirring mechanism to run unevenly and to skip, even in cases where the coils are supplied with sinusoidal alternating current instead of a pulse-shaped alternating current. The occurrence of such phenomena lowers the maximum speed and limits maximum rotary torque levels, and also adversely affects operating reliability, especially with the sort of load fluctuations which are encountered when chemical reactions are taking place in the medium.
In another known design (Swiss patent document No. CH 617 360 A5) an attempt is made to eliminate uneven running by fitting a fixed plate of soft magnetic material such as Mumetal, for example, to the intended axis of rotation of the stirring mechanism and in its immediate vicinity. This solution achieves some desirable characteristics at the expense of economy since the soft magnetic plate tends to shield the permanent magnet of the stirring mechanism. In addition, the distance between the soft magnetic plate and the stirring mechanism has to be kept as small as possible to achieve a marked effect, and this design is unsuitable when a medium has to be stirred in a vessel with a central outlet at the bottom.
The effectiveness of previous designs also suffers from the fact that the electromagnets employed are fitted with members, cores or pole pieces arranged close to one another and a large part of the magnetic field lines of force are thus prevented from reaching the desired effective area.
Previous designs therefore suffer from the fact they are only of limited applicability, that they show uneven rotation or that they have a high magnetic loss. In addition they often require a large spatial volume and complex and expensive construction.